Generally a surface of a magnetic recording layer of a magnetic recording tape such as an audio tape, a video tape or a tape for computers is finished to be smooth to improve sensitivity (outputs particularly in the high frequency band). However, such a smooth magnetic recording tape winds irregularly during winding-up and during rewinding. As a result, running properties are deteriorated due to fluctuation of tension strength, outputs fluctuate and the tape tends to deform or to be damaged.
To remove the above defects, a magnetic tape having a backing layer on the nonmagnetic support on the opposite surface to a magnetic layer has been developed and has been put into practice. Various resin compositions for the binder used in the backing layer have been suggested, such as (1) a mixture of a vinyl chloride-vinyl acetate copolymer, a polyurethane resin, and a polyisocyanate, (2) a mixture of a vinyl chloride-vinyl acetate copolymer, a fiber resin, a polyurethane resin, and a polyisocyanate, and (3) a mixture of a fiber resin, a polyurethane resin, and a polyisocyanate. See, for example, JP-A-57-208635 and JP-A-57-208636 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). In the above-described binder compositions, polyurethane resins are always contained therein as a main component. Polyurethane resins are generally poor in hydrolysis resistance. Accordingly, such binder compositions suffer from the defect that the polyurethane resins hydrolyzed in the backing layer is adhered to the magnetic layer, thereby causing head clogging while a tape is running, and as a result, recording and reproducing cannot be done.
When polyurethane resins are used as a binder, there is also the defect that the friction coefficient increases and the backing layer readily tends to be worn out.
In a magnetic recording medium having a backing layer to improve running characteristics and running durability, the surface of the backing layer is often made rough to improve running, because of the emphasis on running characteristics. As a result, when a magnetic recording medium in the shape of a tape is wound to a rolled state, and is stored for a long period of term, and when a magnetic recording medium in the shape of a sheet is piled or stacked and is stored for a long period of term, the unevenness on the surface of the backing layer is printed onto the surface of the magnetic layer and the surface property of the magnetic layer deteriorates. For the above reason, electromagnetic characteristics, particularly S/N characteristics of the magnetic recording medium deteriorate.